The invention relates to a device for converting the rotary motion of an eccentric driven by a motor shaft into a reciprocating motion of a working tool coupled with a shaft pin in electrically driven appliances, in whose housing a double-armed lever provided with a shaft pin is pivotably mounted.
German Pat. No. 2,409,592 teaches a dry shaver with a rotary motor, an oscillating lower cutter, and a crank and rocker linkage for converting the rotary motion of the motor into the oscillating motion of the lower cutter, whereby the motor shaft is aligned perpendicular to the pivot axis of the rocker arm engaging the lower cutter and is disposed laterally displaced by the length of the crank arm. The crank arm is linked at one end via an elastic gimbal mount with the eccentric on the motor shaft and at the other end via a ball joint with the rocker arm. This type of motion transmission is very expensive and extremely difficult to manufacture because of the tolerances to be maintained for the bearings and articulated parts. Increased play or slippage in the ball joint necessarily produces chattering in the ball joint, while less play results in an increased energy requirement.
Japanese disclosure document No. 57-54141 teaches a dry shaver with a rotary motor, an oscillating lower cutter, and a double-armed rocker arm, wherein the motor shaft of the rotary motor and the bearing of the rocker arm lie on a common axis and the eccentric mounted on the motor shaft directly engages the rocker arm. To convert the rotary motion of the motor shaft into an oscillating motion of the rocker arm, the eccentric pin as well as the bearing pin of the rocker arm engage matching recesses in the rocker arm with relatively considerable play. Because of the continuous changes in position, this results not only in considerable chattering but also considerable frictional losses, which have a disadvantageous effect as a result of increased energy consumption in appliances powered by replaceable or rechargeable batteries.
In addition, the considerable bearing play between the bearing pin and the rocker arm causes the lower cutter fastened to the rocker arm to move away from the cutting foil, thus having a highly disadvantageous effect on the cutting action between the lower cutting and the cutting foil.
Japanese Utility Model No. 57-60788 teaches a dry shaver with a rotary motor, an oscillating lower cutter, and a crank and rocker linkage to convert the rotary motion of the motor into an oscillating motion of the lower cutter. The motor shaft is aligned perpendicular to the pivot axis of the rocker arm engaging the lower cutter, and is disposed displaced laterally by the length of the crank arm. The crank arm is made fork-shaped in the vicinity of the rocker arm, whereby the rocker arm engages this fork and is held in a pivotable position in the fork by means of an articulation pin. The crank arm is also connected by an eccentric bearing provided in the head of the connecting rod with the motor shaft of the rotary motor. An important disadvantage of this type of transmission is that the swing of the rocker arm is compensated by the crank arm only with the accompaniment of considerable losses to pressure and friction.
The goal of the present invention is to develop a device of the species recited hereinabove wherein conversion of the rotary motion into a reciprocating motion is accomplished with minimum possible losses in driving energy. The device is intended to ensure unimpeded motion in all required degree of freedom with little friction, and to operate with low noise. In addition the device must be so designed that both the arrangement of the motor shaft of the rotary motor and the mounting of the double-armed lever on a common axis and on axes staggered with respect to one another are possible.
German OS No. 29 10 469 teaches a swing frame with a linearly oscillating swing beam connected to two drive pins with a lower cutter and fastened to the housing by two pairs of flexible strips. The connecting rod which sets the swing beam in oscilating motion is part of an additional swing frame, partially composed of flexible strips and formed on the swing beam. The pressure exerted by the user when shaving is transmitted directly to the swing beam via the lower cutter coupled to said beam, and thus to the oscillating system, thereby exerting a disadvantageous influence of its oscillating behavior.